thwarted terror plot close to our own border. Canadian police arrested two people allegedly planning to carry out a similar attack to the boston marathon bombing. Using pressure cooker bombs to explode just as the boston marathon bombers did. Pierre thomas has the details from washington this morning. Good morning, pierre. Reporter: Good morning, elizabeth. They're canadians, self-radicalized. No overseas ties or to the u.S., but inspired by al qaeda. Police say they put pressure cooker bombs in a location where there would be a large crowd on canada, which is like our upcoming independence day. Al qaeda has been promoting that kind of bomb on the internet as a weapon of choice, elizabeth. Pierre, I imagine this holiday weekend, given that the boston bombers were going to do their attack this holiday weekend, that security can be tight along the east coast cities. Reporter: That's right. U.S. Officials are taking no chances. Last night, the fbi and homeland security put out a bulletin, urging billions. They are saying even though they identified no specific threat, they're concerned about self-radicalized lone wolves just like the couple in canada, which are hard to detect. Expect to see stepped-up security in a number of big cities, where there will be large celebrations, boston, new york, washington. As you said, extra vigilance if you see anything. Pierre, thank you so much. Understandable.

This transcript has been automatically generated and may not be 100% accurate.

Now Playing: Terror Plot Foiled in Canada

Now Playing: Deadly terror attack on streets of Barcelona

Now Playing: Two people in custody in connection with Barcelona attack

Now Playing: Lost engagement ring turns up in family's garden after 13 years

Now Playing: {{itm.title}}

{"id":19562909,"title":"Pressure Cooker Bomb Plot Thwarted on Canada Day","duration":"1:21","description":"Security has been heightened in the US after Canadian police arrested two for alleged terror plan.","url":"/GMA/video/canada-terror-plot-thwarted-pressure-cooker-bomb-plot-19562909","section":"GMA","mediaType":"default"}